Certain throwing weapons are usable by classes that do not get the throwing skill, e.g.
Shattering Hammers
If you use throwing weapons as a non-throwing class, you will get skillup messages and can skillup throwing up to 275 (which is higher than even the rogue throwing skillcap of 250) although your throwing skill does not show up in your skills tab.
And yes, getting skillups with this bug will improve your throwing damage and hit rate.